I. Field
The subject technology relates generally to communications systems and methods, and more particularly to systems and methods that determine and perform analog gain step magnitude tracking and compensation.
II. Background
Wireless receivers that can be found in cell phones or other wireless devices employ various components to ensure fidelity of reception for received wireless signals. On the front end of these receivers are typically an antenna followed by a filter which is then followed by a low noise amplifier (LNA) to provide analog gain and to boost or reduce the respective signal accordingly depending on the strength of the signal received from the antenna. Output from the LNA is then passed to other intermediate processing components including an analog to digital converter (A/D) before being processed by a digital variable gain amplifier (DVGA). An AGC block controls both analog and digital components to ensure that the output signal level is constant. In general it is highly desirous to hold output power from the DVGA which is also the output from the AGC as constant as possible in order that the signal provided to the rest of the wireless receiver can be processed accordingly.
Typically, the AGC controls the output power of the signal by modifying analog and digital gains. The analog gain applied is controlled by gain adjustment signals from the AGC block. The analog gain can take several discrete values, which correspond to different analog gain states. The difference in the magnitude of the analog gain from one state to the next is called the analog gain step magnitude. Thus, if a change in signal strength were detected that was of large enough magnitude, a command is directed from the DVGA to the LNA, or the mixer, or both to change the gain state from one gain level or state to the next. After an analog gain state has been commanded to switch up or down, the DVGA can provide adjustments in order to attempt to hold the output power of the AGC as constant as possible. In order to command gain state changes, some thresholds have to be overcome before an actual gain state change is initiated by the DVGA. Hysteresis margins can be included in these thresholds to prevent unnecessary switching from one state to the next and back again in a short period of time which can have a negative impact on AGC performance.
One problem with the above approach relates to uncertainty associated with the analog gain step magnitudes. The gain step magnitude uncertainty originates partially from component mismatches within the AGC. Temperature variation is also known to have an impact on the gain step magnitude, which introduces an additional dynamic component of the uncertainty. The nominal gain step magnitude values are stored in AGC. The mismatch of the nominal gain step magnitude and the actual value creates at least the following problems: (1) reduction of hysteresis margin, leading to unnecessary switching of analog gain state, degrading receiver performance (2) saturation of the DVGA output, and (3) reduction of SQR (signal-to-quantization ratio) at the A/D output. A potential approach is to calibrate the analog gain steps for mobile devices in factory to compensate for component tolerances. However, an undesirable consequence is the individual calibration of every mobile device, which increases cost thus incurred. Moreover, the gain step magnitude uncertainty caused by temperature variation cannot be calibrated in advance.